Virtual controller for mixed air low temperature protection of HVAC systems

ABSTRACT

A method for protecting an HVAC system of an air handling unit from low temperature outdoor ventilation air. The air handling unit may include an outdoor fresh air region, a return air region, a supply air region, and a damper situated in or adjacent to the fresh air region to regulate the flow of outdoor air into the air handling unit. The air handling unit mixes the outdoor fresh air and the return air to provide a mixed air stream to the HVAC system. In one illustrative embodiment, one or more sensors are used to measure the temperature and flow rate of the air entering or passing through the outdoor fresh air region, the temperature of the air entering or passing through the return air region and the flow rate of the air passing through the supply air region. The temperature of the mixed air stream, which is provided to the HVAC system, is then calculated using a controller or the like. In some cases, when the temperature of the mixed air stream falls below a threshold value, the controller may instruct the damper to close and reduce the fresh outdoor air that is brought into the air handler unit. Various other embodiments and algorithms are disclosed.

FIELD

The present invention relates generally to Heating, Ventilation, and Air Conditioning (HVAC) systems, and more particularly to methods and apparatus for regulating the flow of outdoor air to help protect the HVAC systems from low temperature air.

BACKGROUND

Modern buildings have an HVAC system that is used to control the environment of the building's inside space. In many systems, air from the building's inside space is drawn into return ducts and provided back to the HVAC system. To meet desired ventilation requirements, many HVAC systems include an exhaust port for exhausting at least some of the return air to the outside environment, and an intake port for bringing in fresh air to the HVAC system. In some cases, a damper is provided that selects how much return air is exhausted and how much outside air is brought into the building. Thus, and depending on the conditions, the air entering the rooms is often a mixture of fresh outdoor air and return air.

In some systems, an HVAC economizer is provided to act as a first stage of cooling to increase fuel economy of the HVAC system during some cooling cycles. The HVAC economizer may mix cooler outdoor air and warmer return air to provide essentially “free” cooling during some or all cooling cycles. At times of heating or when the outdoor air temperature is unsuitably low, the economizer may automatically enter a “lockout” position. The lockout position may hold the outdoor air damper at a closed position or minimum outdoor airflow setting to prevent the undesirably low temperature air from entering the HVAC system.

In many HVAC economizer applications, a low temperature controller is installed in the mixed air stream. The low temperature controller limits the amount of cooler outdoor airflow when the mixed air temperature drops below a safe operating low temperature limit of the HVAC system. If the mixed air temperature is too low, it may cause condensation or freezing in the HVAC cooling and/or heating coils, which in many cases, is undesirable. In the case of modular or packaged air handling unit components, it is difficult to install low temperature protection sensors and control. In some cases it may be physically impossible to install an averaging or other sensor in the mixed air stream plenum of many HVAC economizers. In other cases, the economizer outdoor air dampers, airflow station, and controller are fabricated and shipped as an enclosed module, which makes it difficult or impossible to install a mixed air low temperature sensor/controller. Thus, a method and system for determining the mixed air temperature to help protect the HVAC system from undesirably low temperatures would be desirable in these and other applications.

SUMMARY

The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.

The present invention relates generally to Heating, Ventilation, and Air Conditioning (HVAC) systems, and more particularly to methods and apparatus for regulating the flow of outdoor air to help protect the HVAC systems from undesirably low temperature air. In one illustrative embodiment, an Air Handling Unit (AHU) may be provided to determine a characteristic of a mixed air stream, wherein the AHU includes at least two flow inputs and a flow output. In order to determine a characteristic of the mixed air stream such as mixed air temperature, predetermined characteristics of the first flow input, predetermined characteristics of the second flow input and predetermined characteristics of the flow output may be measured or otherwise determined. The characteristic of the mixed air stream may then be calculated as a function of these predetermined characteristics, as desired. In some cases, the predetermined characteristics may include air temperature, air flow, air humidity, dew point and/or some other characteristic or characteristics, as desired.

In one example, the air in the first flow input may be fresh outdoor air, the air in the second flow input may be return air, and the air in the flow output may be a supply air. The AHU may measure or otherwise determine the temperature and air flow of the fresh outdoor air, the temperature of the return air, and the air flow of the supply air. From this, the AHU may calculate the air temperature of the mixed air stream that passes to the HVAC cooling coils of the HVAC system. It is contemplated that various combinations of air temperature and air flow of the fresh outdoor air, return air, supply air may be used to compute the temperature of the mixed air stream, as desired. In some cases, if the temperature of the mixed air stream falls below a predetermined threshold, the AHU may begin to close the damper that controls the air flow of the fresh outdoor air to help increase the temperature of the mixed air stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative Air Handling Unit (AHU) having an HVAC system for use with a building;

FIG. 2 is a flow diagram of an illustrative mixed air temperature calculation method;

FIG. 3 is a flow diagram of another illustrative mixed air temperature calculation method;

FIG. 4 is a flow diagram of another illustrative mixed air temperature calculation method;

FIG. 5 is a flow diagram of another illustrative mixed air temperature calculation method;

FIG. 6 is a flow diagram of an illustrative calculation of the supply airflow rate;

FIG. 7 is a flow diagram of an illustrative method for protecting an HVAC system from undesirably low temperatures; and

FIG. 8 is a legend that defines the parameters used in the flow diagrams of FIGS. 2-7.

DETAILED DESCRIPTION

The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The detailed description and drawings show several embodiments which are meant to be illustrative of the claimed invention.

FIG. 1 is a schematic diagram of an illustrative air handling unit (AHU) 16 in a building 20. The building 20 may be a residential, commercial, or any other suitable building, as desired. The AHU 16 may include a heating, ventilation, and air conditioning (HVAC) unit 40, which in some cases, may include one or more cooling and/or heating coils. In the illustrative embodiment, the AHU 16 includes at least two inputs and one output. A first input may correspond to the fresh outdoor air input 32. The temperature and/or flow rate of the fresh outdoor air input may be measured or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic, such as humidity, dew point, carbon dioxide level, etc., of the fresh outdoor air input 32 may be measured and/or determined, as desired.

A second input to the AHU 16 may correspond to the return air input 30. The return air input 30 may include air that is pulled from the rooms inside of the building 20. In some cases, some of the return air may be exhausted as shown at 38 through a damper 29, and some of the return air may be recirculated back into the HVAC 40. The temperature and/or flow rate of the return air input 30 may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the return air may be measured and/or determined as desired.

A mixed air stream 34 may correspond to the AHU output. The mixed air stream 34 may be a mixture of the fresh outdoor air input 32 and the return air input 30. The temperature and/or flow rate of the mixed air stream 34 may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the mixed air stream 34 may be measured and/or determined as desired. The HVAC system 40 may include a fan 18 to induce flow of air through the HVAC 40 and ductwork, as desired, to produce supply air 36 to the building 20. In some cases, the temperature and/or flow rate of the supply air 36 may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the return air may be measured and/or determined as desired.

In some illustrative embodiments, a damper 28 may be provided to regulate the flow of fresh outdoor air 32 into the building 20. Likewise, a damper 29 may be provided to regulate the amount of return air that is exhausted 38 from the building 20. Yet another damper 31 may be provided to regulate the flow of return air 30 to mix with the fresh outdoor air 32. In many cases, the dampers 28, 29 and 31 may be mechanically coupled together so that the dampers 28 and 29 open and close together or in sequence, and damper 31 opens and closes in an opposite manner to dampers 28 and 29. Thus, when damper 28 is opened to allow more fresh outdoor air into the building, damper 29 also opens to allow a similar amount of return air to be exhausted from the building. In this example, the return air damper 31 may close as the dampers 28 and 29 open. This arrangement may help balance the pressure inside the AHU 16.

In some cases, the dampers 28 and 29 and associated duct work may be provided in an economizer, shown generally at 50. Under some conditions, the economizer 50 may provide a first stage of “free” cooling by mixing cooler fresh outdoor air 32 with the sometimes warmer return air 30 to provide a mixed air stream 34 to the cooling coils of the HVAC system 40. If the temperature of the mixed air stream 34 is too low, it may cause condensation or freezing in the HVAC cooling and/or heating coils, which in many cases, is undesirable. Thus, the present invention may include a controller 54 that calculates the temperature and possibly other characteristics of the mixed air stream 34 and, by adjusting the damper 28 and sometimes dampers 29 and 31, limits the low temperature of the mixed air stream 34 that is provided to the HVAC cooling and/or heating coils.

In some cases, the AHU 16 may also include a heat exchanger generally shown at 52. The heat exchanger 52 may be adapted to efficiently transfer heat energy between the incoming fresh outdoor air 32 and the exhausted air stream 38, which may be useful under some operating conditions.

FIG. 2 is a flow diagram of an illustrative mixed air stream 34 temperature calculation method. To perform the illustrative mixed air stream 34 temperature calculation, sensor data is first acquired, as shown at block 60. In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow 36 sensor (SplyFlow) is acquired by the controller 54 (see FIG. 1). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow 36 sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream 34, if desired. Once this data is acquired, and as shown at block 62, the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function: Mixed  Air  Temp = (Outdoor  Air  Temp * Outdoor  Air  Flow + Return    Air  Temp * (Supply  Flow − Outdoor  Air  Flow)) * (1/Supply  Flow)

Note that when other characteristics of the various flow streams are measured, different characteristics of the mixed air flow stream may be calculated. For example, if an outdoor dew point sensor (OAD) and a return air dew point sensor (RAD) are provided, a Mixed Air Dew Point (MAD) value may be determined using a similar illustrative function: Mixed  Air  Dew  Point = (Outdoor  Air  Dew  Point * Outdoor  Air  Flow + Return  Air  Dew  Point * (Supply  Flow − Outdoor  Air  Flow)) * (1/Supply  Flow) Instead of using dew point sensors, humidity and temperature sensors may be used to calculate the Outdoor Air Dew Point and the Return Air Dew Point, if desired. Also, Mixed Air Carbon Dioxide levels (MACD), as well as many other mixed air parameters may be determined in a similar manner, if desired.

In any event, the illustrative mixed air temperature (MAT) calculation may be used to help protect the HVAC system 40 from low outdoor air 32 temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block 64. The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow 32 may be reduced, as shown at block 66. The fresh outdoor airflow 32 may be reduced by, for example, adjusting the damper 28 position to reduce the incoming fresh outdoor airflow 32. In some cases, the damper 29 position may be equally decreased and the return air damper 31 may be increased to help balance the pressure within the HVAC system 40.

FIG. 3 is a flow diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream 34 temperature calculation, sensor data is first acquired, as shown at block 70. In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and return air flow sensor (RAFlow) is acquired by the controller 54 (see FIG. 1). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and return air flow (RAFlow) may be provided at convenient locations outside of the mixed air flow stream 34, if desired. Once this data is acquired, and as shown at block 72, the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function: Mixed  Air  Temp = (Outdoor  Air  Temp * Outdoor  Air  Flow + Return    Air  Temp * Return  Air  Flow) * (1/(Outdoor  Air  Flow + Return  Air  Fow)) The illustrative mixed air temperature (MAT) calculation may then be used to help protect the HVAC system 40 from low outdoor air 32 temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block 74. The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow 32 may be reduced, as shown at block 76. The fresh outdoor airflow 32 may be reduced by, for example, adjusting the damper 28 position to reduce the incoming fresh outdoor airflow 32. In some cases, the damper 29 position may be equally decreased and the return air damper 31 may be increased to help balance the pressure within the HVAC system 40.

FIG. 4 is a logic diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream 34 temperature calculation, sensor data is first acquired, as shown at block 80. In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and supply airflow sensor (SplyFlow) is acquired by the controller 54 (see FIG. 1). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and supply airflow sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream 34, if desired. Once this data is acquired, and as shown at block 82, the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function: Mixed  Air  Temp = (Outdoor  Air  Temp * (Supply  Flow − Return  Air  Flow) + Return  Air  Temp * Return  Air  Flow) * (1/Supply  Flow) The illustrative mixed air temperature (MAT) calculation may then be used to help protect the HVAC system 40 from low outdoor air 32 temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block 84. The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow 32 may be reduced, as shown at block 86. The fresh outdoor airflow 32 may be reduced by, for example, adjusting the damper 28 position to reduce the incoming fresh outdoor airflow 32. In some cases, the damper 29 position may be equally decreased and the return air damper 31 may be increased to help balance the pressure within the HVAC system 40.

FIG. 5 is a logic diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream 34 temperature calculation, sensor data is first acquired, as shown at block 90. In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and return air exhaust air flow sensor (RAExhaustFlow) is acquired by the controller 54 (see FIG. 1). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and return air exhaust air flow sensor (RAExhaustFlow) may be provided at convenient locations outside of the mixed air flow stream 34, if desired. Once this data is acquired, and as shown at block 82, the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function: Mixed  Air  Temp = (Outdoor  Air  Temp * Return  Air  Exhaust  Flow + Return  Air  Temp * Return  Air  Flow) * (1/(Return  Air  Flow + Return  Air  Exhaust  Flow)) This function assumes that that the return air exhaust airflow (RAExhaustFlow) is approximately equal to the outdoor airflow (OAFlow), and thus dampers 28 and 29 preferably move together in this illustrative embodiment. Also, return air damper 31 may open and close in an opposite manner to dampers 28 and 29.

The illustrative mixed air temperature (MAT) calculation may be used to help protect the HVAC system 40 from low outdoor air 32 temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block 94. The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow 32 may be reduced, as shown at block 96. The fresh outdoor airflow 32 may be reduced by, for example, adjusting the damper 28 position to reduce the incoming fresh outdoor airflow 32. The damper 29 position may equally or substantially equally decrease the return air exhaust airflow (RAExhaustFlow), and the return air damper 31 may increase the return air flow 30 accordingly.

FIG. 6 is a flow diagram of an illustrative method for determining the supply airflow (SplyFlow) 36, as shown at block 100. The illustrative method may be run on a regular basis, such as every second, minute, hour, day, etc., and may be used to set and/or update the supply airflow (SuplyFlow) parameter.

The supply airflow (SplyFlow) 36 may depend on the type of HVAC system 40 used. Block 110 determines if a constant volume system is used. If a constant volume system is used, control is passed to block 112. Block 112 determines if an outdoor airflow station is present, which provides a measure of the outdoor air flow (OAFlow). If an outdoor air flow station is present, control is passed to block 114. Block 114 determines if the damper 28 of the economizer is at the full open (>=100%) position. If the damper 28 is at the full open (>=100%) position, control is passed to block 116, which updates a MAXFLOW parameter with the outdoor air flow (OAFlow) that is currently measured by the outdoor airflow station. Control is then passed to block 120, which updates the supply air flow (SplyFlow) with the updated MAXFLOW parameter.

Referring back to block 114, if the damper 28 is not at the full open (>=100%) position, control is passed to block 120, which updates the supply air flow (SplyFlow) with the old MAXFLOW parameter.

Referring back to block 112, if an outdoor air flow station is not present in the system, or is otherwise not working, control is passed to block 118. Block 118 updates the supply air flow (SplyFlow) with the designed flow rate of the system (DsgFlow) multiplied by a Dirty Filter Factor (DirtyFltrFctr). The Dirty Filter Factor (DirtyFltrFctr) may be a value ranging from one (clean) to zero (very dirty), and may provide a measure of the reduction in supply air flow caused by the HVAC filter. From blocks 118 and 120, control is passed to block 136, wherein the algorithm is exited.

Referring back to block 110, if a constant volume system is not used, the system must be a Variable Air Volume (VAV) system, and control is passed to block 122. Block 122 determines if the system is a Variable Air Volume (VAV) HVAC system that includes a Supply Flow Station for providing a measure of the supply air flow. If so, control is passed to block 124. Block 124 receives the current supply air flow (SplyFlow) from the Supply Flow Station. Control is then passed to block 136, wherein the algorithm is exited.

Referring back to block 122, if a Variable Air Volume (VAV) HVAC system 40 is used that does not include a Supply Flow Station, control is passed to block 126. Block 126 determines if the Variable Air Volume (VAV) HVAC system 40 includes an outdoor air flow Station that provides a measure of the outdoor air flow (OAFlow). If so, control is passed to block 128. Block 128 determines if the economizer damper 28 is at the full open (>=100%) position. If the damper 28 is not at the full open (>=100%) position, control is passed to block 134. If the damper 28 is at the fill open (>=100%) position, control is passed to block 130, which updates the MAXFLOW parameter with the outdoor air flow (OAFlow) currently measured by the outdoor airflow station. Control is then passed to block 134.

Block 134 updates the supply air flow (SplyFlow) as a function of the flow capacity signal of the Variable Air Volume system, the minimum air flow setting of the Variable Air Volume system times a dirty filter parameter, and the MAXFLOW parameter. Control is then passed to block 136, wherein the algorithm is exited.

Referring back to block 126, if the Variable Air Volume (VAV) HVAC system 40 does not includes an outdoor air flow Station that provides a measure of the outdoor air flow (OAFlow), control is passed to block 132. Block 132 calculates the supply air flow (SplyFlow) as a function of the flow capacity signal of the Variable Air Volume system, the minimum air flow setting of the Variable Air Volume system, the MAXFLOW parameter, and a dirty filter parameter. The flow capacity signal Page: 11 _([0])typical ranges from 0 to 100%, depending on the cooling demand of the zone terminals. Control is then passed to block 136, wherein the algorithm is exited.

FIG. 7 is a flow diagram of an illustrative method of protecting an HVAC system from undesirably low temperatures. It is contemplated that the illustrative method shown in FIG. 7 may be run on a regular basis, such as every second, minute, hour, day, etc.

To perform the illustrative method, sensor data is first acquired as shown at block 200. In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow 36 sensor (SplyFlow) is acquired by the controller 54 (see FIG. 1). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow 36 sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream 34, if desired. Once this data is acquired, and as shown at block 210, the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function: Mixed  Air  Temp = (Outdoor  Air  Temp * Outdoor  Air  Flow + Return    Air  Temp * (Supply  Flow − Outdoor  Air  Flow)) * (1/Supply  Flow) Control is then passed to block 220. Block 220 determines if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Heat Exchanger Low Limit (HtgExchgLow) or a Low Comfort Limit (LowComfort). If so, control is passed to block 222. Block 222 limits the outdoor air flow ventilation that is provided. The fresh outdoor airflow 32 may be reduced by, for example, adjusting the economizer damper 28 position to reduce the incoming fresh outdoor airflow 32. In some cases, the damper 29 position may be equally decreased to help balance the pressure within the HVAC system 40. Control is then passed to block 224, which issues a diagnostic warning signal or message.

Referring back to block 220, if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are not less than the Heat Exchanger Low Limit (HtgExchgLow) or the Low Comfort Limit (LowConfort), control is passed to block 230. Block 230 determines if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Safety Limit. If not, control is passed to block 240, wherein the algorithm is exited.

If the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Safety Limit, then control is passed to block 232. Block 232 starts a delay timer. Control is then passed to block 234. Block 234 determines if the Delay Timer has exceeded a timer limit. If the Delay Timer has not exceeded the timer limit, control is passed to block 240, wherein the algorithm is exited. The timer limit allows the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) to be less than a Safety Limit for a predetermined period of time, which may help reduce nuisance fan stops, as further described below.

If the Delay Timer has exceeded the timer limit, control is passed to block 236. Block 236 stops the fan system and closes the outside air damper 28. Control is then passed to block 238, which issues an alarm signal or message. Control is then passed to block 240, wherein the algorithm is exited. FIG. 8 is a legend that defines the parameters used in the flow diagrams of FIGS. 2-7.

Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention's scope is, of course, defined in the language in which the appended claims are expressed. 

1. A method for determining a characteristic of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes at least two flow inputs that are mixed to provide the mixed airflow stream, and a flow output, the method comprising: providing a measure of a first characteristic and a second characteristic of air in a first flow input; providing a measure of the first characteristic of air in a second flow input; providing a measure of the second characteristic of air in the flow output; and calculating the first characteristic of the mixed air stream in the AHU as a function of the first and second characteristics of the first flow input, the first characteristic of the second flow input, and the second characteristic of the flow output.
 2. The method of claim 1 wherein the first characteristic is temperature and the second characteristic is air flow.
 3. The method of claim 1 wherein the first characteristic is moisture content and the second characteristic is air flow.
 4. The method of claim 1 wherein the first characteristic is carbon dioxide concentration and the second characteristic is air flow.
 5. The method of claim 2 wherein the first input is a fresh outdoor air input.
 6. The method of claim 5 wherein the second input is a return air input.
 7. The method of claim 6 wherein the flow output is a supply air output.
 8. The method of claim 1 wherein the AHU includes a heating, ventilation, and air conditioning (HVAC) system.
 9. The method of claim 1 wherein the measure of the first characteristic and the second characteristic are provided by one or more sensors.
 10. A method for determining a characteristic of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes at least two flow inputs that are mixed to provide the mixed airflow stream, and a flow output, the method comprising: providing a measure of a first characteristic and a second characteristic of air in a first flow input; providing a measure of the first characteristic and the second characteristic of air in a second flow input; calculating the first characteristic of the mixed air stream in the AHU as a function of the first and second characteristics of the first flow input and the first and second characteristics of the second flow input.
 11. The method of claim 10 wherein the first characteristic is temperature and the second characteristic is air flow.
 12. The method of claim 10 wherein the first characteristic is moisture content and the second characteristic is air flow.
 13. The method of claim 10 wherein the first characteristic is carbon dioxide concentration and the second characteristic is air flow.
 14. The method of claim 11 wherein the first input is a fresh outdoor air input.
 15. The method of claim 14 wherein the second input is a return air input.
 16. The method of claim 15 wherein the flow output is a supply air output.
 17. The method of claim 10 wherein the AHU includes a heating, ventilation, and air conditioning (HVAC) system.
 18. The method of claim 10 wherein the measure of the first characteristic and the second characteristic are provided by one or more sensors.
 19. A method for determining the temperature of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes an outdoor fresh air input port, a return air input port, a return air exhaust port, and a supply air port, wherein the mixed airflow stream is a mixture of the air passing through the outdoor fresh air input port and the return air input port, the method comprising: providing a measure of temperature and air flow for air passing through the outdoor fresh air input port; providing a measure of temperature for air passing through the return air input port; providing a measure of air flow through the supply air port; and calculating a temperature for the mixed air stream in the AHU as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow through the outdoor fresh air input port and the supply air port.
 20. The method of claim 19, wherein the AHU further includes a damper for controlling the air flow through the outdoor fresh air input port, the method further comprising the steps of: adjusting the damper to reduce the air flow through the outdoor fresh air input port if the temperature of the mixed air stream falls below a predetermined threshold temperature value.
 21. The method of claim 20 wherein the adjusting step only occurs if the temperature of the mixed air stream falls below the predetermined threshold temperature value for a predetermined time period.
 22. A method for determining the temperature of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes an outdoor fresh air input port, a return air input port, a return air exhaust port, and a supply air port, wherein the mixed airflow stream is a mixture of the air passing through the outdoor fresh air input port and the return air input port, the method comprising: providing a measure of temperature for air passing through the outdoor fresh air input port; providing a measure of temperature and air flow for air passing through the return air input port; providing a measure of air flow for air passing through the return air exhaust port; calculating a temperature for the mixed air stream in the AHU as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow flowing through the return air input port and the return air exhaust port.
 23. An air handling unit (AHU) having an outdoor fresh air input port, a return air input port, and a supply air port, wherein the AHU mixes the air passing through the outdoor fresh air input port with the return air input port to produce a mixed air stream, the AHU comprising: a first temperature sensor positioned in the outdoor fresh air input port for providing a measure of the temperature of the air passing through the outdoor fresh air input port; a first air flow sensor positioned in the outdoor fresh air input port for providing a measure of the air flow of the air passing through the outdoor fresh air input port; a second temperature sensor positioned in the return air input port for providing a measure of the temperature of the air passing through the return air input port; a second air flow sensor positioned in the supply air port for providing a measure of air flow through the supply air port; and a controller coupled to the first temperature sensor, the second temperature sensor, the first air flow sensor and the second air flow sensor, the controller adapted to calculate a temperature for the mixed air stream as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow passing through the outdoor fresh air input port and the supply air port.
 24. The air handling unit (AHU) of claim 23 further comprising a damper for controlling the air flow through the outdoor fresh air input port.
 25. The air handling unit (AHU) of claim 24 wherein the controller is adapted to adjust the damper to reduce the air flow through the outdoor fresh air input port if the temperature of the mixed air stream falls below a predetermined threshold temperature value.
 26. The air handling unit (AHU) of claim 25 wherein the controller is further adapted to only adjust the damper if the temperature of the mixed air stream falls below the predetermined threshold temperature value for a predetermined time period.
 27. A method for determining the mixed air temperature in a air handling unit (AHU), the air handling unit including a fresh air region, a return air region, and a supply air region, the method comprising: determining the supply airflow rate in the supply region; sensing the outdoor air temperature in the fresh air region; sensing the outdoor airflow rate in the fresh air region; sensing the temperature of the return airflow in the return air region; and calculating the mixed airflow temperature as a function of the supply airflow rate, the outdoor air temperature, the outdoor airflow rate, and the temperature of the return airflow.
 28. The method of claim 27, wherein the supply airflow is calculated.
 29. The method of claim 27, wherein the supply airflow is measured.
 30. The method of claim 27 further comprising the steps of: comparing the outdoor air temperature and the mixed air temperature to a threshold temperature; and adjusting an outdoor air damper to regulate the outdoor airflow into the AHU if the outdoor air temperature and/or the mixed air temperature fall below the threshold temperature.
 31. A method for protecting a HVAC system of an air handling unit from low temperatures wherein the air handling unit includes a fresh air region, a return air region, a supply air region, and a damper situated in or adjacent to the fresh air region to regulate the flow of outdoor air into the HVAC, the method comprising: determining the flow rate of the supply air in the supply air region; sensing the outdoor air temperature in the fresh air region; sensing the outdoor airflow rate in the fresh air region; sensing the temperature of the return airflow in the return flow region; calculating the temperature of the mixed air in the air handling unit as a function of the supply airflow rate, the outdoor air temperature, the outdoor airflow rate, and the temperature of the return airflow; comparing the outdoor air temperature and the mixed air temperature to a threshold temperature; and closing the position of the damper to decrease the flow of outdoor air into the air handling unit when the outdoor air temperature and the mixed air temperature are less than the threshold temperature.
 32. The method of claim 31 further comprising issuing a warning signal, an alarm, and/or a message when the outdoor air temperature and the mixed air temperature are less than the threshold temperature.
 33. The method of claim 31 further comprising the steps of: counting the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature with a timer; comparing the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature to a predetermined limit of time allowable for the mixed air temperature and outdoor air temperature to be below the threshold temperature; and closing the position of the damper when the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature is greater than the predetermined limit of time allowable for the mixed air temperature and outdoor air temperature to be below the threshold temperature. 